The instant invention relates to L-shaped mixing heads used in reaction injection molding (RIM) apparatus for combining a plurality of reactive molding components prior to their delivery into a mold. These reactive molding components typically comprise a polyahl and an isocyanate moiety, as those terms are used and described in U.S. Pat. No. 4,390,645 to Hoffman et al, which disclosure is hereby incorporated herein by reference.
The prior art teaches L-shaped RIM mixing heads, or "L-heads", comprising a body with a first bore wherein reactive molding components are impingement mixed; and a second bore intersecting the first bore into which the resulting mix product is ejected upon reciprocation of a plunger within the first bore. The second bore, which is typically of substantially greater diameter than the first bore, defines a "quieting chamber" wherein the velocity of the mix product stream is significantly abated prior to exiting the mixing head. The length of the quieting chamber is calculated, for example, so as to obtain splashless pours into an open mold. A plunger is reciprocated within the second bore at the end of the "shot" to eject remaining mix product from the quieting chamber.
One problem associated with known L-heads is the adherence and subsequent accretion of reactive molding components, either individually or as mixed, to the walls of the quieting chamber. The accreted material tends to radially deflect the second plunger as it reciprocates within the quieting chamber, with an attendant increase in the amount of friction generated between the plunger and the chamber walls. Such increased frictional contact frequently results in the galling of either the plunger or the chamber walls, or both of them, and the ultimate fracture of the plunger is not uncommon. The likelihood of galling is further increased by the length of the quieting chamber demanded by certain high-volume RIM applications.
Should the surface of the plunger become galled, its continued reciprocation past the lip seal of the plunger's hydraulic actuator results in the rapid deterioration of the lip seal and the subsequent contamination of the mix product/molded part with hydraulic fluid. The radial deflection of the plunger also permits reactive material to flow upstream to pack the lip seal, thereby reducing lip seal effectiveness and further encouraging the breach thereof.
The accretion of reactive material on the walls of the quieting chamber further provides increased resistance to plunger reciprocation, whereby mixing head cycling time is increased and part production slowed.
The prior art has heretofore approached the problem of material accretion on the walls of the quieting chamber by specifying closer tolerances for both plunger and bore to reduce the radial gap therebetween. This in turn reduces the amount of material which is able to accrete on the walls prior to being scraped therefrom by the free end of the plunger. While this approach succeeds in increasing the useful life of the plunger and the mixing head body, the closer plunger and bore tolerances required by this approach significantly increase the machining costs associated with those parts, particularly given the length of the quieting chamber required for some applications; and this approach nonetheless relies upon the periodic, albeit less frequent, replacement of the plunger and/or the mixing head body. Thus, this approach results not only in a substantial increase in the cost of mixing head parts but also fails to satisfactorily eliminate the costly production downtime accompanying their ultimate replacement.
The addition of fillers to the mix product further reduces the effectiveness of the prior art approach. Specifically, the passage of filled or "reinforced" mix product through the quieting chamber abrades both the chamber walls and the tip of the plunger. As a result, increasingly less reliance may be had over time on the close fit between the plunger and the bore, and the scraping action of the plunger tip against the bore, to prevent the deleterious accretion of reactive material upon the walls of the quieting chamber.
Another problem encountered with known L-heads is their susceptibility to failure due to misalignment of a plunger with its corresponding bore within the mixing head body. Known mixing heads typically employ hydraulic actuators for reciprocating the first and second plungers in their respective bores. The hydraulic actuators are normally bolted onto the mixing head body so as to facilitate replacement of any given part. Each hydraulic actuator and, hence, the plunger reciprocated thereby are vulnerable to misalignment with their corresponding bore in the mixing head body as a result of either the improper mating of the hydraulic cylinder with the mixing head body, or the mishandling of the mixing head on the shop floor. Still further, the substantial stroke of the second plunger required to clear the quieting chamber of reactive material at the end of each shot amplifies the effect of any plunger-to-bore misalignment, thereby increasing the likelihood of mixing head failure due to galling or seizing of the plunger within the second bore.
The prior art approach is once again the specification of closer tolerances for the mating surfaces, with the increased machining costs associated therewith, to ensure the proper alignment of actuator and bore upon initial mating of each hydraulic actuator with the mixing head body. The problem of subsequent misalignment of plunger and bore on the shop floor has heretofore remained unresolved.